Insulin's interaction with its specific receptors in the plasma membrane of the adipose cell, the initial event in the regulation of glucose transport and metabolism by insulin, is complex. The mechanism of this interaction is under investigation through 1) the development of a mathematical model of 125I-insulin binding and degradation, and 2) an examination of the role of 125I-insulin and insulin receptor internalization in the overall binding process. To date, a one binding site model of insulin's interaction with the adipose cell has been rejected although some of the kinetics of insulin association can be successfully explained by hormone degradation. While a two binding site model can explain the kinetics of both association and dissocation at a given insulin concentration, the parameters of each site or the proportion of sites with constant parameters must vary with insulin concentration. A rapid and insulin concentrationdependent internalization of insulin receptors in the adipose cell has now been demonstrated. Internalized receptors appear in the same low-density microsomal membrane fraction with which the cell's intracellular pool of glucose transport is associated and from which glucose transport systems are translocated to the plasma membrane in response to insulin.